Interferometric encoders can perform high precision measurements of the position of mechanical stages with respect to a reference frame. For this purpose, an encoder head is mounted to the stage and a periodic grating structure (e.g., an encoder scale) is mounted to the reference frame, or vice versa. Compared to free-space distance-measuring interferometers, this can have the advantage of significantly shortened paths in air and thereby reducing the impact of air turbulence. Often, stage motion has to be monitored in multiple dimensions, thereby requiring multiple measurement beams that typically use different diffraction orders of the grating.
In applications of interferometric encoders where space is very limited and stages are accelerated at several tens of m/s2 (e.g., in photolithography steppers), there is an incentive to keeping components of the metrology solution as compact and light weight as possible. With regards to the encoder head, one way to reduce size and weight is to choose a small beam size and thereby enable a scaled-down version of the same encoder head. Consequently, however, the encoder's sensitivity to small scale grating errors is increased since the averaging effect over the beam size is reduced. Furthermore, this approach may, depending on the beam geometry, limit the measurement range along the grating normal over which the encoder can maintain a strong interference signal (e.g., measurement beams may shear and lose overlap with the reference beam).